The invention relates to a preparation method of a biological agent for treating high-concentration phenolic wastewater, belonging to the field of wastewater treatment. The method comprises the following steps: soaking porous coconut shells in a biogas solution, carbonizing to obtain high-adsorptivity cocoanut charcoal particles, rotting the cocoanut charcoal, wood blocks and nitrophenol together to load the microbes generated during wood rotting onto the cocoanut charcoal, carrying out fermentation modification on the microbe-loaded cocoanut charcoal, silt, retinervus luffae fructus and gallic-acid-rich cornus together, and carrying out freeze-drying to obtain the biological agent. After the biological agent is added into the phenolic wastewater according to the addition amount of 300-400 mg/L, the phenol concentration in the wastewater is lowered from 1200-1500 mg/L to 10 mg/L or below, so the removal rate can reach 99.1% or above.